1. Field of the Invention
The present invention relates to a gear mechanism of a power transmitting system that is favorably used as a balancer apparatus of an internal combustion engine.
2. Discussion of Related Art
As well known in the art, in a balancer apparatus of an internal combustion engine, a balance shaft provided with an unbalance weight is operatively coupled with a crankshaft via a gear mechanism, whereby rotational force of the crankshaft is transmitted to the balance shaft. In the balancer apparatus, the balance shaft rotates in synchronization with the crankshaft, whereby inertial force generated by reciprocation of an engine piston is cancelled, and vibration of the engine is accordingly reduced.
Since explosive combustion in the internal combustion engine takes place intermittently, the magnitude of the rotational force transmitted from the crankshaft to the balance shaft is not constant or fixed, but rather is always fluctuating.
The inventors have confirmed that, among frequency components included in the fluctuations of the rotational force, a secondary component of a fundamental frequency that results from engine combustion occurring once in every two rotations of the crankshaft, and a sextic component that is amplified by torsional resonance of the crankshaft are relatively large compared to a component (primary component) of the fundamental frequency that is determined according to the speed of rotation of the crankshaft.
The balancer apparatus receives the rotational force including the vibration components of different frequencies as described above, and therefore vibration occurs in the gear mechanism, in particular, in a meshing portion(s) of the gears. Such vibration may result in generation of noise and reduction in the durability of the gears.
Thus, a balancer apparatus has been proposed wherein a damping mechanism formed by, for example, a spring or springs is inserted in a rotational-force transmission path from the crankshaft to the balance shaft so as to damp the vibration components of the rotational force.
In order to effectively damp a high-frequency component of the fluctuations in the rotational force, such as the sextic component of the fundamental frequency, by using the damping mechanism, the spring constant of the spring(s) must be set to a sufficiently low value so as to reduce the natural frequency of a vibration system formed by the balancer apparatus. However, if the spring constant is merely set to a low value, the spring(s) may be excessively deformed in response to a rapid increase in the rotational force transmitted from the crankshaft upon, for example, acceleration of the engine. Thus, the damping mechanism may be damaged due to the deformation. Moreover, characteristics of the spring may be substantially lost by so-called bottoming or the like, whereby the damping mechanism may cease to function properly.
In view of the above situation, a balancer apparatus in which a damping mechanism provides non-linear spring characteristics has been proposed in, for example, Japanese Laid-Open Patent Publication No. 60-192145.
FIG. 22 shows a cross-sectional structure of a main part of one example of the balancer apparatus. As shown in FIG. 22, the balancer apparatus includes a rotary shaft 100 operatively coupled with a balance shaft (not shown), and a generally cylindrical gear 110 that surrounds the outer periphery of the rotary shaft 100 and operatively coupled with a crankshaft (not shown). The rotary shaft 100 has a plurality of radially protruding driving pieces 102 formed on its outer periphery. The gear 110 also has a plurality of radially protruding driving pieces 112 formed on its inner periphery so as to be located between the corresponding driving pieces 102 of the rotary shaft 100.
Damper chambers 120 are formed between the respective driving pieces 102 of the rotary shaft 100 and the corresponding driving pieces 112 of the gear 110, and an elastic member 130 is disposed in each damper chamber 120. Moreover, clearances 132 are formed between each elastic member 130 and the corresponding driving pieces 102 and 112. In the balancer apparatus thus constructed, the driving pieces 102 and 112 and the elastic members 130 form the damping mechanism.
The operation of the damping mechanism will be now described. As the rotary shaft 100 rotates relative to the gear 110, the clearances 132 are reduced, and the driving pieces 102 and 112 then abut on the respective elastic members 130. As the rotary shaft 100 further rotates relative to the gear 110, the elastic members 130 are elastically deformed, thereby generating elastic force according to the amount of relative rotation. This elastic force (more specifically, torque based on this elastic force) acts against the relative rotation between the rotary shaft 100 and the gear 110.
Referring to FIG. 23, the solid line indicates the relationship between the angle xcex8r of the relative rotation between the rotary shaft 100 and the gear 110 and the elastic force (torque) T. The two-dot chain line indicates the relationship between the relative rotational angle xcex8r and the elastic force T in a comparative example. In the comparative example, the clearances 132 are not formed, and the natural frequency of the vibration system is reduced merely by setting the spring constant of the elastic members 130 to a low value.
As indicated by the solid line of FIG. 23, when the relative rotational angle xcex8r is within a predetermined rotational phase range or angle (xcex8r less than xcex81), the elastic members 130 are not elastically deformed, whereby the elastic force T is xe2x80x9czeroxe2x80x9d. Thus, by forming the clearances 132 between each driving piece 102, 112 and the corresponding elastic members 130 so as to provide a relative rotational phase range in which the elastic force T is not produced, the natural frequency of the vibration system formed by the balancer apparatus can be reduced without significantly reducing the spring constant of the elastic members 130.
When the rotary shaft 100 and the gear 110 rotate relative to each other beyond the predetermined rotational angle (xcex8r greater than xcex81), the elastic force T increases with the relative rotational angle xcex8r. As compared with the comparative example, the relative rotational angle xcex8r is limited to a relatively small value even when the elastic force T becomes extremely large (T=Tmax), that is, when the rotational force transmitted from the crankshaft to the balancer apparatus becomes extremely large (xcex8max1 less than xcex8max2). Thus, the elastic members 130 are not excessively deformed.
Thus, according to the balancer apparatus, a high-frequency component of the fluctuation in rotational force can be damped without causing any damage and deterioration in the function of the damping mechanism when the rotational force from the crankshaft rapidly increases upon, for example, acceleration of the engine.
Such a damping mechanism having a non-linear spring characteristic can certainly reduce the natural frequency of the vibration system formed by the balancer apparatus, and damp the high-frequency component of the fluctuations in the rotational force, while avoiding any damage and deterioration in the function of the damping mechanism.
However, the reduction in the natural frequency of the vibration system may cause a problem as follows: the natural frequency is reduced to be equal to a frequency that is close to that of a low-frequency component, such as the secondary component of the fundamental frequency of the engine, which is included in the fluctuations in the rotational force. As a result, a resonance phenomenon occurs in the balancer apparatus due to the low-frequency component of the fluctuations in rotational force. Thus, vibration resulting from the resonance phenomenon cannot be prevented.
The aforementioned problem occurs not only in the above-described balancer apparatus of the internal combustion engine, but generally occurs in a gear mechanism of a power transmitting system that transmits rotational force by using gears.
The present invention has been developed in the light of the above situations. It is therefore an object of the present invention to provide a gear mechanism of a power transmitting system that is capable of favorably preventing or reducing the occurrence of a resonance phenomenon due to high-frequency and low-frequency components.
To accomplish the above object, the present invention provides for example a gear mechanism of a power transmitting system comprising first and second rotating members disposed coaxially with each other, and a damping mechanism interposed therebetween, wherein the damping mechanism includes a damping member that generates damping force for limiting relative rotation between the first and second rotating members, and at least one elastic member that elastically deforms mainly when an angle of relative rotation between the first and second rotating members exceeds a predetermined rotational angle, so as to apply elastic force onto the rotating members in a direction opposite to that of the relative rotation.
The gear mechanism as described above may be applied to a construction comprising a first gear operatively coupled with a first rotary shaft and meshing with a second gear provided on a second rotary shaft, wherein the second gear forms one of the rotating members and the second rotary shaft forms the other rotating member.
With the gear mechanism constructed as described above, the elastic force of the elastic member is generated mainly when the two coaxially disposed rotating members (the second gear and the second rotary shaft in the above application) rotate relative to each other beyond the predetermined rotational angle. Therefore, the natural frequency of a vibration system including the gear mechanism can be reduced without significant reduction in the spring constant of the elastic member. As a result, a high-frequency component of fluctuations in rotational force can be damped while avoiding any damage and degradation in the function of the damping mechanism. When the two coaxially disposed rotating members rotate relative to each other within the predetermined rotational phase range or angle, on the other hand, the damping member generates damping force for limiting the relative rotation between the rotating members. Therefore, the damping capability of the damping mechanism can be enhanced, and a low-frequency component of the fluctuations in the rotational force can also be damped.
Thus, even where the rotational force including both low-frequency and high-frequency components as vibration components is transmitted to the gear mechanism constructed according to the present invention, occurrence of a resonance phenomenon due to the low-frequency and high-frequency components can be advantageously prevented without causing any damage or degradation in the function of the damping mechanism.
In one preferred form of the present invention, the damping member comprises a friction damping member that is located between the two rotating members, so as to generate the damping force in the form of friction force that arises due to relative rotation between the two rotating members.
With the gear mechanism constructed as described above, the damping force does not significantly change with a change in the speed at which the two rotating members rotate relative to each other, and may be held substantially constant. Therefore, the capability of damping, in particular, a low-frequency component of the fluctuations in rotational force can be improved as compared with a structure that uses a so-called oil damper or the like as the damping member. As a result, occurrence of a resonance phenomenon due to the low-frequency component can be further advantageously suppressed or prevented.
In a further preferred form of the present invention, each of the at least one elastic member comprises a main deformation portion arranged at an acceleration side of the elastic member that elastically deforms mainly when the two rotating members rotate relative to each other beyond the predetermined rotational angle as the rotational force transmitted between the two rotating members increases or is kept substantially constant, and a sub deformation portion arranged at a deceleration side of the elastic member that elastically deforms mainly when the two rotating members rotate relative to each other beyond the predetermined rotational angle as the rotational force transmitted between the two rotating members decreases, the acceleration-side elastic portion having a greater limit to elastic deformation thereof than that of the deceleration-side elastic portion.
With the gear structure constructed as described above, the acceleration-side elastic portion elastically deforms when the two rotating members rotate relative to each other beyond the predetermined rotational angle not only in the case where the rotational force increases but also in the case where the rotational force is kept substantially constant. Thus, the acceleration-side elastic portion functions to transmit the rotational force between the two rotating members. Therefore, the acceleration-side elastic portion is more frequently subjected to elastic deformation than the deceleration-side elastic portion.
In view of the above, the damping mechanism of the invention may be constructed such that the main deformation portion has a greater limit to elastic deformation thereof than that of the sub deformation portion. Thus, the main deformation portion is allowed to elastically deform to a greater extent, thus assuring improved durability of the elastic member(s).
In the above preferred form of the invention, the main deformation portion and the sub deformation portion may be formed of a rubber material, and the main deformation portion may have an elastically deformable portion whose volume is larger than that of an elastically deformable portion of the sub deformation portion.
At least one of the two rotating members may comprise a gear meshing with a respective counter gear.
In the gear mechanism of the present invention, at least one of the gear and the counter gear may be a resin gear whose teeth comprise a resin material.
In the above structure, impact that acts on a meshing portion between the meshing gears is absorbed, and gear meshing noise can be thus reduced. Furthermore, the fluctuations in the rotational force transmitted between the gears, in particular, its high-frequency component, can be advantageously damped.
Also, in the case where the other of the meshing gears that meshes with the resin gear is a metal gear, the gear meshing noise can be reduced even if the working accuracy of a tooth surface of the metal gear is relatively low. This makes it possible to eliminate some process steps, such as shaving and polishing the tooth surface of the metal gear, and backlash control by selection and adjustment of a shim commonly used for forming metal gears. Moreover, since resin gears are respectively meshed with metal gears in the gear mechanism, disadvantages such as thermal adhesion between the gears can be avoided.
In the gear mechanism as described above, one of the meshing gears may be a resin gear whose teeth are formed of a resin material, while the other of the meshing gears may be a metal gear whose teeth are formed of a metal, and the resin gear may have a tooth width that is larger than that of the metal gear.
In the case where the resin gear and the metal gear mesh with each other, respective tooth positions of the gears may be displaced from each other in the tooth-width direction of the gears due to an error in mounting the gears, vibration during rotation, and the like. In such a case, only a local portion of the tooth surface of the resin gear abuts on the tooth surface of the metal gear, resulting in so-called local abutment. Since the resin gear is generally less wear-resistant and less durable than the metal gear, the resin gear may further be worn and/or damaged as a result of the local abutment.
In view of the above point, the gear mechanism of the present invention may be constructed such that the resin gear has a tooth width that is larger than that of the metal gear. Therefore, even if the respective tooth positions of the gears are displaced from each other, the displacement is covered, and abutment of local portions of the resin and metal gears is avoided. As a result, otherwise possible wear and damage of the resin gear resulting from the displacement can be suppressed or prevented.
In the gear mechanism as described above, at least one elastic member may be provided on one of the two rotating members, and at least one abutting member may be provided on the other of the two rotating members, each abutting member abutting on the corresponding elastic member to cause elastic deformation thereof when the two rotating members rotate relative to each other beyond the predetermined rotational angle. In this arrangement, the strength of each abutting member as measured upon breakage of the abutting member due to elastic force of the corresponding elastic member acting thereon may be set to be smaller than the strength of a toothed portion of one of the meshing gears that is formed as the resin gear.
In the case where at least one of the gears is a resin gear, a toothed portion of the resin gear may be broken when it receives excessive rotational force since the strength of the resin gear is lower than that of a metal gear. This may result in a problem such as biting of the gears.
In this respect, the gear mechanism of the invention may be constructed such that the abutting member is broken prior to breakage of the toothed portion of the resin gear, whereby mechanical coupling force between the two rotating members is rapidly reduced. With this arrangement, the breakage of the toothed portion of the resin gear upon receipt of the excessive rotational force is avoided, whereby the problem such as biting of the gears can be prevented in advance.
In another preferred form of the present invention, the damping mechanism comprises a plurality of elastic members as the above-indicated at least one elastic member, each of which is provided on one of the two rotating members, and a plurality of abutting members corresponding to the respective elastic members are provided on the other of the two rotating members, for abutting on the corresponding elastic members to cause elastic deformation thereof when the two rotating members rotate relative to each other beyond the predetermined rotational angle. Furthermore, the elastic members are located with respect to the one of the two rotating members such that different angles of relative rotation between the two rotating members are formed when the respective elastic members successively abut on the corresponding abutting members.
With the above arrangement, the elastic members as a whole exhibit an even more non-linear elastic characteristic when the two rotating members rotate relative to each other. Therefore, the natural frequency of the vibration system including the gear mechanism is dispersed or diversified into a plurality of frequencies, whereby the damping capability of the damping mechanism is further enhanced. As a result, occurrence of the resonance phenomenon can be suppressed in a further preferable manner.
In a further preferred form of the invention, the damping mechanism comprises a plurality of elastic members, each of which is provided on one of the rotating members, and a plurality of abutting members corresponding to the respective elastic members, each of which is provided on the other of the rotating members for abutting on the corresponding elastic members to cause elastic deformation thereof when the rotating members rotate relative to each other beyond the predetermined relative rotational angle, wherein the elastic members and the abutting members are located with respect to the rotating members such that the elastic members and the abutting members are spaced from each other at equal intervals in a direction of rotation of the rotating members, and wherein the number of teeth of the gear being coupled by the gear mechanism is set to an integral multiple of the number of the elastic members.
Thus an increased degree of freedom with which the gear is mounted may be achieved.
In a further preferred form of the invention, the predetermined relative rotational angle is defined by the sum of angles by which each abutting member is spaced from corresponding end faces of the above-indicated at least one elastic member which face the abutting member, as viewed in a direction of rotation of the rotating members.
The present invention may be applied to an internal combustion engine comprising a crankshaft, at least one balance shaft and a gear mechanism according to the invention, wherein the at least one balance shaft is driven by a rotational torque of the crankshaft.
In a further preferred form of the invention, the gear mechanism is arranged at the first balance shaft and comprises a driven gear being disposed on the first balance shaft and rotatable relative thereto, and wherein the driven gear is driven by a crank gear being fixedly secured on the crankshaft.
In a still further preferred form of the invention, the gear mechanism is arranged at the crank shaft and comprises a crank gear being disposed on the crankshaft and rotatable relative thereto, and wherein the crank gear drives a driven gear being fixedly secured on the first balance shaft.
The internal combustion engine may comprise a second balance shaft being operatively coupled with the first balance shaft.
In a further form of the invention, the second balance shaft may be driven by the crankshaft via the crank gear, an intermediate gear being disposed on an intermediate shaft and meshing with the crank gear, a driven gear being disposed on the second balance shaft and rotatable relative thereto and meshing with the intermediate gear, and an additional gear mechanism connecting between the driven gear and the second balance shaft.